1. Field of Invention
Method and apparatus for facilitating time based cryptographic encoding and decoding.
2. Discussion of Related Art
The notion of “sending a secret message to the future” has been around for over a decade. Despite this, no solution to this problem is in common use, or even attained widespread acceptance as a fundamental cryptographic primitive. Sending a message into the future was first proposed by Timothy May, in “Time-release crytpo”, since then many protocols have been proposed to encrypt messages to be sent into the future, usually under a name like “timed-release cryptography”. These known methods provide only estimates of or lower bounds on elapsed time.
Solutions that do not have a fixed decryption time generally involve expensive sequential computations (“time-lock puzzles”—Merkle is generally credited with inventing these “puzzles”) to recover an initial message, ensuring that the recipient cannot recover the data before some length of time. Other solutions that do not guarantee fixed time release are made possible by partial key escrow.
Some methods use known encryption techniques in which the decryption key is kept secret until a fixed revelation time. The problem has been described as a “Timed Release Encryption Problem” as a sender encrypting a message such that only a particular receiver can decrypt that message, and that only after a specific release time has passed, as verified by a single trusted, third-party time server. This solution uses a bilinear pairing on a Gap Diffie-Hellman group, which requires reasonable cryptographic assumptions. This solution is similar to those employed in identity-based cryptography. Other works sharing this connection is known as “secure timed-release public key encryption” and its equivalence to strongly key-insulated public key encryption. The solution, also based on a bilinear map, requires a trusted “timed-release public server” that periodically publishes information, based on a private secret, that enables decryption of previously encrypted texts. Other proposals include a related protocol in which digital signatures become verifiable only at a fixed future time t upon publication by a trusted third party of “some trapdoor information associated with the time t.”
Other methods use “token-controlled” public key encryption. In token-controlled encryption, messages are encrypted with both a public encryption key and a secret token, and can only be decrypted with the private decryption key after the token is released.
In addition to time-lock puzzles, a similar system uses a secret decryption key and a trusted third party to create and distribute public and private keys at appropriate times. Another work uses a trusted time server and a new primitive called “conditional oblivious transfer” to send messages into the future where the server never learns the senders identity, however it does learn the receiver's identity.